A conventional radio communication system (wireless LAN communication system) will be explained. At present, products according to the IEEE 802.11b standard, the IEEE 802.11a standard, and the like, which are standardized according to the American IEEE 802.11 Wireless LAN Standards (see Non-Patent Literature 1: IEEE 802.11 from http//standards.ieee.org/getieee802/802.11.html) have been marketed as apparatuses for constructing home/office high-speed wireless network systems.
A wireless LAN according to the IEEE 802.11b standard (see Non-Patent Literature 2: IEEE 802.11b) has a maximum physical transmission rate of 11 megabits per second, using a 2.4-gigahertz band and complementary code keying (CCK) as a modulation scheme. A wireless LAN according to the IEEE 802.11a standard (see Non-Patent Literature 3: IEEE 802.11a) has a maximum physical transmission rate of 54 megabits per second, using a 5-gigahertz band and orthogonal frequency division multiplex (OFDM) as a modulation scheme. A wireless LAN according to the IEEE 802.11g standard, for which specifications of the standard are being considered, has a maximum physical transmission rate of 54 megabits per second, using a 2.4-gigahertz band and the ODFM as a modulation scheme.
The conventional radio communication systems have, however, a problem in that an effective rate indicating at what rate a data stream can be actually transmitted is often equal to or lower than half the maximum physical transmission rate.
Specifically, a data stream to be transmitted, for example, is divided into a plurality of data packets. Each data packet is added with header information including information for transmission control including destination/sender IP addresses, a packet length, a packet number, and the like and with information for error correction control. The data packets added with the information are received by a lower layer as international protocol (IP) packets. In a media access control (MAC) layer, a data frame is also added with header information including information for transmission control including destination/sender MAC addresses, a frame length, and the like, as well as information for error correction control and the data frame may be encoded and added with decoding information to be received by a physical layer. In the physical layer, the data frame is added with header information including information for transmission control including a modulation scheme, a frame length, and the like, as well as a preamble for synchronization and the like to be transmitted.
Furthermore, the base station or each radio terminal performs carrier sensing for the radio channel before transmission of the radio frame. If the base station or radio terminal confirms that the channel is being used (the channel is busy), it refrains from transmitting the radio frame. After confirming that the channel is not being used (the channel is idle), the base station or radio terminal uses a random access scheme called carrier sense multiple access/collision avoidance (CSMA/CA) for transmitting the radio frame. A base station or a radio terminal designated by the MAC address returns an ACK/NACK frame indicating whether the radio data frame has been correctly received. If the radio data frame has not been correctly received, the frame is retransmitted.
Accordingly, the effective rate is not equal to the physical transmission rate for the wireless LAN according to the IEEE 802.11b, IEEE 802.11a, or IEEE 802.11g standard. Actually, therefore, the effective rate is equal to or less than approximately half the physical transmission rate, depending on the environmental conditions of the transmission system.
Namely, if the conventional home/office wireless network system (wireless LAN) according to the IEEE 802.11a standard, the IEEE 802.11b standard, the IEEE 802.11g standard, or the like is to perform bidirectional communications for a data stream of a video signal for a high resolution television HDTV (High Definition Television) that requires, for example, approximately 20 megabits per second, the effective rate is insufficiently low.
To solve the problem of the insufficient effective rate, there is proposed, for example, the following method disclosed in Japanese Patent Application Laid-Open No. 2002-135304. In this method, if a broadband data stream, for example, is to be transmitted and received, IP packets are allocated to a plurality of radio units operating with different channels to be transmitted and received under independent controls of the respective radio units. However, if the respective units use different modulation schemes or the allocated IP packets have different sizes, a delay is caused by processes such as rearrangement of packets, because the allocation to the radio units is carried out in IP packet units. Furthermore, the leakage power from an adjacent channel becomes higher than a carrier sense threshold because of the independent controls of the respective radio units. As a result, normal transmission cannot be carried out.
There is also proposed the following different method. In this different method, one radio unit serves as a master, and if a broad transmission band is necessary for video transmission or the like, a sub radio unit corresponding to a channel allocated in advance is operated as a slave. The master transmits and receives a control signal for a plurality of radio units to acquire a radio channel access right, whereby the radio units transmit and receive IP packets. This method has, however, the following problem similarly to the above method. When the radio units use different modulation schemes or the allocated IP packets have different sizes, because the allocation to the radio units is carried out in IP packet units, on one hand, reception cannot be performed even if a radio unit has completed transmission, if another radio unit has not completed transmission. On the other hand, a terminal receiving IP packets cannot perform transmission even if a radio unit has completed reception, if another radio unit has not completed reception. As a result, the radio band cannot be efficiently used.
The present invention has been achieved in view of the above problems. It is an object of the present invention to provide a radio communication system (a base station and a radio terminal) capable of improving the throughput by efficiently using the radio band.